1. Field of the Invention
The present invention relates to a method for manufacturing a solid-state imaging device of a charge coupled device (CCD) type, a metal oxide semiconductor (MOS) type, or a complementary metal oxide semiconductor (CMOS) type. More specifically, the present invention relates to a method for manufacturing a solid-state imaging device employing a hole accumulation (accumulated) diode (HAD) structure as a technique for preventing dark current that possibly arises in a sensor part and the periphery thereof.
2. Description of the Related Art
A solid-state imaging device (referred to also as an image sensor), such as a CCD image sensor and a CMOS image sensor, including plural charge generators (sensor parts) each formed of a photoelectric conversion element (such as a photo diode) in an imaging unit is being used as a device for capturing an image in various fields.
In a general solid-state imaging device, the respective light-receiving elements that each serve as major part of the sensor part (light-receiving part) and are each formed of a photo diode or the like receive incident light through the light-receiving planes and carry out photoelectric conversion. The generated charges are detected by a detection circuit and then are amplified so as to be sequentially output.
As one configuration example of the solid-state imaging device, on an N-type silicon substrate (semiconductor substrate of a first conductivity type), a P-type impurity layer (P-well) as a semiconductor layer of a second conductivity type is formed. Furthermore, a sensor part (light-receiving part) including a charge accumulation layer (hereinafter, referred to also as a first sensor region) formed by ion implantation of an impurity of the first conductivity type into the semiconductor layer of the second conductivity type is formed. Signal charges obtained through light reception and photoelectric conversion are accumulated in this charge accumulation layer.
FIG. 9 is a diagram for explaining the occurrence of dark current in a configuration without a HAD structure. FIG. 10 is a diagram for explaining an advantage of a HAD structure formed by ion implantation in terms of suppression of the dark current. It is known that, in the solid-state imaging device, crystal defects in a photo diode and, as shown in FIG. 9, interface states at the interface between the photo diode and an insulating film thereon act as the sources of the dark current. As schemes for suppressing the occurrence of the dark current attributed to the interface states, e.g. a buried photo diode structure and a HAD structure are known.
The buried photo diode is obtained by forming a semiconductor region of a first conductivity type (e.g. n-type) (hereinafter, this region will be referred to as an n-type semiconductor region) and forming a shallow, heavily-doped semiconductor region of a second conductivity type (p-type) for dark current suppression (hereinafter, this region will be referred to as a hole accumulation region) on the surface of this n-type semiconductor region, i.e. in the vicinity of the interface with the insulating film. In a general method for fabricating the buried photo diode, ion implantation of boron (B) or boron fluoride (boron difluoride (BF2)) serving as the p-type impurity and annealing treatment (heat treatment) are performed to thereby fabricate the p-type semiconductor region in the vicinity of the interface between the n-type semiconductor region of the photo diode and the insulating film.
As shown in FIG. 10, the HAD structure is obtained by stacking a hole accumulation layer (hereinafter, referred to also as a second sensor region) formed of a P+-type impurity region on a charge accumulation layer formed of an N+-type impurity region on the surface side of the NP diode. The following description will deal with the HAD structure formed by ion implantation.
In a configuration without the HAD structure like that shown in FIG. 9, electrons generated attributed to interface states flow into the photo diode as dark current. In contrast, the HAD structure like that shown in FIG. 10 can suppress the dark current attributed to the interface states by the hole accumulation layer formed near the interface.
Specifically, the solid-state imaging device of the HAD structure includes a sensor part of a HAD sensor structure having the hole accumulation layer that is stacked on a signal charge accumulation layer for accumulating charges generated depending on incident light in order to enhance the sensitivity and suppress surface dark current. As described above, the signal charge accumulation layer is formed by ion implantation of an N+-type impurity, and the hole accumulation layer is formed by ion implantation of a P+-type impurity. In this sensor part of the HAD sensor structure, the N-type semiconductor layer (signal charge accumulation layer) existing under the hole accumulation layer and the P-type semiconductor layer existing under the N-type semiconductor layer serve as a photo diode that carries out photoelectric conversion. In the solid-state imaging device having such a HAD structure, electrons generated in the vicinity of the substrate surface due to thermal excitation are trapped by the hole accumulation layer, and thus the occurrence of dark current is suppressed, which provides enhanced sensitivity.
On the other hand, there has been proposed a back-irradiation solid-state imaging device as a device having the buried photo diode structure (refer to Japanese Patent Laid-open No. 2003-31785 (hereinafter, Patent Document 1)). To obtain this device, the backside of a silicon substrate in which photodiodes and various transistors are formed is polished to decrease the thickness of the substrate. This allows light entry from the backside of the substrate for photoelectric conversion. As described above, a shallow, heavily-doped p-type semiconductor region (hole accumulation region) is formed in the photo diode part in order to suppress dark current. In the case of the back-irradiation solid-state imaging device, this hole accumulation region is formed on both the front side and the backside of the substrate.